Rotational magnetic sensor

Abstract: A rotary shaft position sensor includes a stator (22) and a rotor (24) and produces a linear output signal in accordance with the rotational position of a shaft (68) coupled to the rotor. The rotor rotates about an axis of rotation. The stator includes stationary pole pieces (42, 44) having stationary pole faces situated perpendicular to the axis of rotation and spaced a predetermined distance apart to define an air gap therebetween. A Hall effect device (50) is situated in the air gap. The Hall device (52) produces an output signal corresponding to the level of magnetic flux in the air gap. One or more magnets (58) are attached to the rotating pole piece (60). The magnetic poles of the magnets are situated parallel with and spaced a predetermined distance from the stationary pole faces. Lines of magnetic flux originating from the magnets are situated in a direction perpendicular to the stationary pole face (54, 56). The rotating pole piece rotates in close proximity to the stationary pole piece (42, 44). As the rotor rotates about the axis of rotation, a portion of the magnet is aligned with the stationary pole faces (54, 56). Magnetic flux corresponding to the aligned portion of the magnet and the stator pole faces appears in the air gap. A temperature compensation circuit for use with the rotary shaft position sensor includes first and second temperature compensation circuits for counteracting sensitivity temperature drift and zero gauss offset temperature drift characteristics of the Hall effect device (50).

Description: Field of the Invention: This invention relates to rotary shaft position sensors and more specifically to a sensor that produces a linear output signal in detecting the rotational position of a throttle valve of an internal combustion engine. Background Of The Invention: Electronic fuel injected engines used in motor vehicles typically include a microprocessor based control system. Fuel is metered, or injector activation time is varied, in accordance with various engine parameters detected through sensors. Some of the parameters include engine temperature, mass air flow rate, present vehicle speed, engine RPM, oxygen content in the exhaust, and, most importantly, the position of the butterfly valve or throttle valve that controls air flow into the engine. Typically the throttle valve rotates from a closed position to an open position in a span of ninety degrees or less. Throttle valve position detection is accomplished by attaching a rotational position sensor to the rotational axis of the throttle valve. The rotational position sensor produces an output voltage signal in accordance with the relative position of the throttle valve. The accuracy and repeatability of the sensor over an extended period of time must not vary significantly. Otherwise, the engine control computer may be required to compensate for drift or variance of the sensor. Software that must compensate for sensor output signal drift can be unduly complex and waste program memory space better used for engine control algorithms. It is preferred that the sensor produce a linear output and that over the life expectancy of the sensor, the output signal versus rotational position vary only an insignificant amount as temperature extremes are encountered. Installation of a high accuracy sensor in the engine compartment of a motor vehicle is a requirement with regard to the throttle valve sensor. Temperature extremes in the engine compartment typically result in temperature drift of electronic circuits installed therein. Linear Hall effect devices are simplistic in operation and may be used with non-contact type magnetic coupling rotary sensors to produce a signal corresponding to angular position. A more reliable throttle position sensor that includes non-contact type operation and Hall effect based electronics would satisfy several concerns of automobile manufacturers good signal to noise ratios are achieved and if the temperature drift of the Hall device is compensated through a low component count temperature compensation circuit. Reducing cost and improving long term accuracy and reliability are the foremost concems in producing a reliable engine compartment sensor. Designers of rotational magnetic sensors have used Hall effect devices therein, and examples of such sensing systems are given in German Patent Nos. 1,281,549 and 1,276,346, as well as European Patent No. 0,578,299 Al. German Patent No. 1,281,549 describes a rotor angle sensing system for an electric meter, wherein a series of opposite polarity magnets formed within the rotor induce magnetic flux of alternating polarity with a Hall effect disposed exposed on a face of the stator. German Patent No. 1,276,346 discloses a magnetic sensor comprising: Hall effect means for sensing magnetic flux and providing an output signal corresponding to a sensed level of magnetic flux; a rotor member arranged to rotate about a rotational axis and to conduct magnetic flux from a rotary surface of said rotor which is perpendicular to said axis of rotation, a stator member arranged to conduct magnetic flux from a static surface perpendicular to said axis of rotation, magnetic means for producing magnetic flux across a first air gap (B) between said rotary and said static surfaces, and the Hall effect means being arranged to sense the level of flux between said surfaces, Finally, European Patent No. 0,578,299 Al discloses a rotor angle sensing system, wherein a portion of a housing defines a magnetic gap between the rotor and stator (Art.54(3,4) EPC document). WO/92/10722 discloses a magnetic sensor of the type defined in the pre-characterising part of claims 1 and 3. EP-B-0 273 103 describes a temperature compensation circuit for a sensor. The circuit can independently compensate both for offset and sensitivity. While each of the foregoing rotor angle sensing systems are operable to produce an electrical signal relating the angle of a rotor relative to a stator, each has drawbacks. A rotary sensor that includes improved signal to noise, a linear output signal response, is more economical to produce and is resistant to temperature extremes is therefore needed. A first aspect of the invention provides a magnetic rotational position sensor according to claim 1. A second aspect of the invention provides a magnetic rotational position sensor according to claim 3. Further embodiments are set out in the claims dependent on the independent claims. Specfic embodiments of the present invention will become apparent from the following drawings and written description. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is an exploded view of a rotational magnetic sensor according to a first embodiment of the present invention. FIG. 1B is a perspective view of the rotational magnetic sensor of FIG. 1A assembled. FIG. 2 is a prior art rotational magnetic sensing arrangement including a geometrically shaped magnet that generates radial lines of magnetic flux. FIG. 3 is a side elevational view of the rotational magnetic sensor of FIGS. 1A and 1B. FIG. 4 is a diagrammatic illustration of the magnetic circuit of the rotational magnetic sensor of FIGS. 1A and 1B. FIG. 5 is a plan view of two 180 degree stationary stator plates of the rotational magnetic sensor of FIGS. 1A and 1B spaced opposite one another to define an air gap therebetween. FIG. 5A is a plan view of alternate 180 degree stationary stator plates of the rotational magnetic sensor of FIGS. 1A and 1B. FIG. 6 is a side elevational view of the stator plates of FIG. 5. FIG. 6A is a side elevational view of the alternate stator plates of FIG. 5A. FIG. 7 is a plan view of a geometrically shaped or contoured magnet of the rotational magnetic sensor of FIGS. 1A and 1B that generates axial lines of magnetic flux. FIG. 8 is a side elevational view of the geometrically shaped magnet of FIG. 7. FIG. 9 is a graph depicting the output voltage waveform of a rotational magnetic sensor having a true, semi-circular shaped magnet. FIG. 10 is a graph depicting the output voltage waveform of the rotational magnetic sensor of FIGS. 1A and 1B. FIG. 11 is a schematic of a temperature compensated Hall effect device amplifier circuit. FIG. 12A is an exploded view of a rotational magnetic sensor according to a second embodiment of the present invention. FIG. 12B is a perspective view of the rotational magnetic sensor of FIG. 12A assembled. FIG. 13 is a side elevational view of the rotational magnetic sensor of FIGS. 12A and 12B. FIG. 14 is a diagrammatic illustration of the magnetic circuit of the rotational magnetic sensor of FIGS. 12A and 12B. FIG. 15A is an exploded view of a rotational magnetic sensor according to a third embodiment of the present invention. FIG. 15B is a perspective view of the rotational magnetic sensor of FIG. 15A assembled. FIG. 16 is a graph depicting the output voltage waveform of the rotational magnetic sensor of FIGS. 15A and 15B. FIG. 17 is a graph depicting the output voltage for the rotational magnetic sensor of FIGS 15A and 15B as a function of its angular position. FIG. 18A is an exploded view of a rotational magnetic sensor according to a fourth embodiment of the present invention. FIG. 18B is a perspective view of the rotational magnetic sensor of FIG. 15A assembled. FIG. 19 is a plan view of a fringing shield that is uses with geometrically shaped or contoured magnets of the rotational magnetic sensor of FIGS. 18A and 18B. DESCRIPTION OF THE PREFERRED EMBODIMENT For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. Referring to FIGS. 1A and 1B, a rotational magnetic sensor 20 is shown. The sensor 20 is used for sensing angular position of an object or shaft and producing an output signal corresponding to the sensed angular position. Sensor 20 includes a stator assembly 22 and a rotor assembly 24 aligned on a central axis 26. Magnetic sensor 20 generates a voltage output signal that is a true linear analog representation of the relative angle between stator assembly 22 and rotor assembly 24. Therefore, magnetic sensor 20 is usable as a position sensor without substantive modification of the voltage output signal to provide linear feedback in the detection of, for example, throttle valve rotational position. Unlike prior art magnetic sensors, magnetic sensor 20 includes a magnet that supplies magnetic flux in a direction parallel, rather than perpendicular, to its axis of rotation, thereby resulting in increased magnetic coupling and improved signal to noise ratios. Referring now to FIG. 2, a prior art rotational magnetic sensing arrangement 30 is shown including a magnet 32 spaced a fixed distance 'A' from a Hall effect device 34. Magnetic lines of flux generated by magnet 32 emanate from pole 36 and are received by pole 38. Rotation of magnet 32 about its central axis 40 changes the spaced relation of the magnetic lines of flux generated by magnet 32 with respect to Hall effect device 34 to induce a corresponding change in output signal generated by Hall effect device 34. Magnet 32 is geometrically shaped to produce a linear change in output signal from Hall effect device 34 as magnet 32 and its radial flux field are rotated about axis 40. Because device 30 is dependent on a precise spatial relationship between magnet 32 and Hall effect device 34 (i.e., dimension A), device 30 is susceptible to any deviation from this spatial relationship as well as to any changes in the magnetic flux field generated by magnet 32, such as those changes resulting from misalignment of the magnet relative to the Hall effect device and magnetic interference caused by the presence of nearby ferrous objects. The present invention represents a significant improvement over prior art arrangement 30 by providing a closely coupled magnetic circuit. The magnetic flux generated by the magnet is parallel to the rotational axis of the sensor and perpendicular to opposing faces of rotor assembly 24 and stator assembly 22. As a result, sensor 20 produces a higher signal to noise ratio and is less susceptible to misalignment. Further, by providing stator pole pieces in close proximity to rotor pole pieces to direct the magnetic flux into the air gap, sensor 20 is less sensitive to the influence of nearby ferrous objects. Referring back to FIGS. 1A and 1B, stator assembly 22 includes, as means for conducting magnetic flux, stationary pole pieces 42 and 44 centered about central axis 26. Pole pieces 42 and 44 include upstanding flanges 46 and 48, respectively, that define an air gap for receiving a Hall effect device therein. The Hall effect device 50 produces an output signal in accordance with the magnetic flux density impinging thereon. Preferably, a Hall effect device 50 is adhesively attached between flanges 46 and 48 to detect the magnetic flux in the air gap and produce an output voltage signal corresponding to that flux. Hall effect device 50 includes connectors 52 for connecting with associated Hall effect circuitry, such as the amplifier circuit shown in FIG. 11. Pole pieces 42 and 44 are constructed of a ferrous material, such as cold rolled steel, in plate form to define pole faces 54 and 56. Faces 54 and 56 oppose rotor assembly 24 and include integral flanges 46 and 48 that are rolled or formed perpendicularly upward from faces 54 and 56. Rotor assembly 24 includes means for generating magnetic flux in the form of a geometrically shaped or contoured magnet 58 attached to pole piece 60 and centered about central axis 26. Pole piece 60 includes a pole face 64 that receives magnet 58 thereon with pole face 66 of magnet 58 supported parallel to face 56 of stationary pole piece 44. Pole piece 60 is formed having a flanged or stepped portion that provides an upper pole face 69 parallel to face 54 of stationary pole piece 42 and flush with face 66 of magnet 58. Unlike prior art magnetic sensors that generate radial lines of flux, magnet 58 generates axial lines of magnetic flux that are directed either toward stationary pole faces 54 and 56 or face 64, as indicated in the direction of the arrows 70. Referring now to FIGS. 3 and 4, rotational magnet sensor 20 is shown defining an air gap 'B' between faces 54 and 56 of stator assembly 22 and faces 66 and 69 of rotor assembly 24. Air gap 'C' is defined by flanges 46 and 48 of stationary pole pieces 42 and 44, respectively In a preferred embodiment, pole pieces 42 and 44 are molded or encapsulated in plastic to provide a rugged mounting arrangement for sensor 20. Pole pieces 42 and 44 are situated in a predefined spatial relationship, for example parallel, with respect to rotor assembly 24. Preferably, Hall effect device 50 is attached between the pole pieces using an adhesive and/or also by encapsulating in plastic. Similarly, rotor assembly 24 is a unitary structure formed by attaching magnet 58 using an adhesive to surface 64 of pole piece 60 and by attaching pole piece 60 to hub 62 of shaft 68. Shaft 68 is representative of an object having a varying angular position that is desirably sensed and represented by a linear analog signal; i.e., throttle valve position. However, sensor 20 is usable as well to sense derivatives of position, such as velocity or acceleration. Since sensor 20 operates by detecting or measuring the magnetic flux coupled through the stator assembly 22 from the rotor assembly 24, rotor assembly 24 must be positioned in fixed relation to stator assembly 22. In the preferred embodiment, precision bearings (not shown) are provided to support rotor assembly 24 both axially and longitudinally along central axis 26 to maintain a constant air gap 'B' and to maintain the rotor faces in alignment with the stator faces. In FIG. 4, a magnetic circuit flow diagram for device 20 is shown in greater detail with lines of magnetic flux shown flowing from magnet 58, through stepped pole piece 60, across air gap 'B', through pole piece 42, across air gap 'C', through pole piece 44 and back across air gap 'B' to magnet 58, thereby completing a closely coupled magnetic circuit. As rotor assembly 24 rotates about axis 26, a variable magnetic flux density is generated across air gap 'C'. The polarity and flux density of the magnetic lines of flux flowing across the air gap is a function of the proportional magnetic surface area alignment between faces 54 and 56 of the stationary pole pieces and faces 66 and 69 of rotor assembly 24. For example, as shown in FIG. 4, face 66 of magnet 58 is aligned with face 56 of stationary pole piece 44 to generate a maximum level of flux density across air gap 'C'. Conversely, when magnet 58 is positioned with equal portions of face 66 aligned with faces 54 and 56 of pole pieces 42 and 44, respectively, a zero magnetic flux density is generated across air gap 'C'. In this zero magnetic flux condition, the magiietic forces are balanced out in the magnetic circuit, with magnetic flux flowing only across air gap 'B'. This position is referred to as a magnetically neutral position. Rotation of rotor assembly 24 in either direction from the neutral position generates a magnetic flux having an assigned polarity (i.e., north or south) across air gap 'C'. Unlike prior art magnetic circuits in which a magnet supplies magnetic flux in a direction perpendicular to its axis of rotation, the magnet in sensor 20 supplies magnetic flux in a direction parallel to its axis of rotation. As such, a varying portion of the magnetic flux generated by magnet 58 is provided into the magnetic circuit as rotor assembly 24 rotates relative to stator assembly 22, thereby yielding a sensor less susceptible to magnetic interference and misalignment. Referring now to FIGS. 5 and 6, stator pole pieces 42 and 44 are shown in greater detail. As shown in FIG. 5, each of stator pole pieces 42 and 44 span an angle D of 180 degrees to define air gap 'C' between matching flanges 46 and 48. As such, magnetic flux flowing from pole piece 42 to 44 is directed between flanges 46 and 48 into air gap 'C'. Alternately, as shown in FIGS. 5A and 6A, flanges 45 and 47 can also be provided at the opposite ends of the 180 degree span symmetric with flanges 46 and 48 to promote increased magnetic coupling between the stator pole pieces. Referring now to FIGS. 7 and 8, magnet 58 spans a 180 degree arc. Although contemplated as having a true semi-circular shape or profile, in the preferred embodiment magnet 58 is geometrically shaped or contoured in radial length to correct for both the magnetic coupling and shunting effects that occur when the rotor assembly is rotated through zero magnetic flux and maximum magnetic flux positions. These magnetic effects can detract from a truly linear analog sensor output as shown, for example, in FIG. 9. As an alternative to geometrically shaping or contouring magnet 58, upper pole face 69 of pole piece 60 may be geometrically shaped or contoured to correct for both the magnetic coupling and shunting effects. In either case, whether upperface 69 or magnet 58 is geometrically shaped, the aligned portion of the stator and rotor assemblies varies in a compensating non-linear fashion as the rotor assembly rotates with respect to the stator assembly to cause a linear output signal to be produced by the Hall effect device. In FIG. 9, the output voltage waveform 80 developed by a sensor having a magnet with a true semi-circular profile, rather than a magnet with a geometrically shaped profile, is shown. Between opposite polarity maximum flux points 82 and 84, output voltage waveform 80 is slightly S-shaped having an inflection point 86 at the zero flux density level. In the vicinity of point 86, the magnet 58 is near its neutral position and magnetic shunting occurs to alter the output voltage waveform; i.e., the measured flux density changes disproportionately in air gap 'C' versus angular position as rotor assembly 24 rotates through its zero flux position. In the vicinity of maximum flux density locations 82 and 84, the face 66 of magnet 58 is nearly fully aligned with one of the stator pole pieces, and magnetic coupling occurs to alter the output voltage waveform; i.e., the measured flux density changes disproportionately in air gap 'C' as rotor assembly 24 rotates through its maximum flux positions. However, this non-linear magnetic interaction between the rotor and stator assemblies is readily solved by slightly modifying or "geometrically" altering the cross-sectional profile of magnet 58 to compensate for the shunting and coupling non-linearities. Referring back to FIGS. 7 and 8, the radius 'R' of magnet 58 is altered as a function of angular position 'P' along the profile of magnet 58 to correct for the above-described non-linear effects. As such, the operative opposing surface area between face 66 of magnet 58 and face 56 of pole piece 44 changes in a non-linear fashion with incremental rotation of rotor assembly 24 to offset the magnetic coupling and shunting effects so that a linear output waveform is produced by the Hall effect device. The particular geometric shape and corresponding pole face area of magnet 58 is application specific and empirically determined based on various magnetic circuit characteristics such as magnet diameter, magnet strength and material, air gap dimensions ('B' and 'C'), desired output gain or slope, desired range of rotational movement, acceptable linearity error, type of Hall effect device and acceptable cost. Nevertheless, one such geometric shape for a magnet constructed from Alnico 8 and having a thickness of 0.376 cm (0.148 inches) and an inner radius of 0.318 cm (0.125 inches) is provided according to the Table 1 in reference to Fig.7; ANGLE (degrees) RADIUS (inches) (cm) P=0-10° R=0.351 0.892 P=10-20° R=0.355 0.902 P=20-30° R=0.359 0.912 P=30-40° R=0.363 0.922 P=40-50° R=0.367 0.932 P=50-60° R=0.371 0.942 P=60-120° R=0.375 0.953 P=120-130° R=0.371 0.942 P=130-140° R=0.367 0.932 P=140-150° R=0.363 0.922 P=150-160° R=0.359 0.912 P=160-170° R=0.355 0.902 P=170-180° R=0.351 0.892 Referring now to FIG. 10, the linear output voltage waveform 90 developed by sensor 20 with geometrically shaped magnet 58 is shown. Between opposite polarity maximum flux points 92 and 94, output voltage waveform 90 is linear, both in the vicinity of zero flux point 96 and maximum flux density points 92 and 94. The geometrically shaped magnet offsets the magnetic shunting effect occurring near point 96 and the magnetic coupling effects occurring near points 92 and 96. Referring now to FIG. 11, a schematic of a temperature compensated Hall effect device amplifier circuit is shown for use with a magnetic sensor incorporating a Hall effect device 102 similar to Hall effect device 50 of sensor 20. Circuit 100 provides basic amplifier/gain control of the output voltage from Hall effect device 102. Because sensor 20 is usable in automotive applications and, therefore, is likely to be subjected to harsh environments, additional circuitry is required to compensate for the effects of temperature variance on the operative characteristics of the Hall effect device. For example, the magnetic flux at which a zero voltage signal is produced by the Hall effect device can change with temperature. Similarly, the sensitivity or gain of the voltage output signal as a function of the sensed magnetic flux can change with the ambient temperature surrounding the Hall effect device. To compensate for these temperature effects, zero magnetic flux density offset voltage drift correction and Hall effect device sensitivity correction are provided by circuit 100. In particular, circuit 100 compensates for operational differences attributable to temperatures ranging from -40 to 150 degrees Celsius typically encountered in an engine compartment of a motor vehicle. Zero gauss offset correction is provided in portion 104 of circuit 100, wherein resistors R1-R6 and diode D1 are connected in a conventional wheatstone bridge arrangement. It is well known that silicon diodes exhibit a predictable variation in forward bias voltage dependent upon temperature. The bias voltage temperature drift (which is substantially linear) provides a correction factor for zero flux offset drift of the Hall effect device 102. The attenuated voltage signal at 105 from circuit portion 104 is supplied to the inverting input of operational amplifier 108. The output voltage signal at 107 from Hall effect device 102 is supplied to the non-inverting input of operational amplifier 110. Operational amplifiers 108 and 110 are conventionally connected to provide an active feedback loop amplifier, wherein the values of resistors R7 and PTC control the total loop gain of the feedback amplifier combination. The positive temperature coefficient resistor (PTC) 111, connected in series with resistor R7, alters the gain of the amplifier 108 in response to changes in ambient air temperature. As a result, circuit 104, which provides the zero gauss temperature offset correction, functions independently of circuit portion 106, which provides temperature correction for the sensitivity level of the Hall effect device. Referring now to FIGS. 12A and 12B, an alternate embodiment of a sensor 120 according to the present invention is shown. Rotational magnetic sensor 120 includes a stator assembly 122 similar to stator assembly 22 of sensor 20. Stator assembly 122 includes stationary pole pieces 142 and 144 centered about central axis 126 of sensor 120. Pole pieces 142 and 144 each include upstanding flanges 146 and 148, respectively, that define an air gap 'C'. Hall effect device 150 is adhesively attached between flanges 146 and 148 and produces an output signal corresponding to the flux density in gap 'C'. Unlike sensor 20, sensor 120 includes a rotor assembly 124 having two magnets, 158 and 159, attached to a planar, rather than stepped, pole piece 160. Pole piece 160 supports magnets 158 and 159 on face 164 with face 166 of magnet 159 situated flush with face 167 of magnet 158, and parallel to pole faces 156 and 154. Magnets 158 and 159 are geometrically shaped or contoured and are attached to pole piece 160 and centered about central axis 126. Similar to magnet 58, magnets 158 and 159 generate axial lines of magnetic flux 169 that are directed perpendicular to stationary pole faces 154 and 156. Referring now to FIG. 13, an air gap 'B' is defined between pole faces 154 and 156 of stator assembly 122 and faces 166 and 167 of rotor assembly 124. An air gap 'C' is defined between flanges 146 and 148 of stationary pole pieces 142 and 144, respectively. Similar to pole pieces 42 and 44 of sensor 20, pole pieces 142 and 144 are situated in a predefined spatial relationship, for example parallel, with respect to rotor assembly 124. Preferably, stationary pole pieces 142 and 144 are molded or encapsulated in plastic to provide a rugged mounting arrangement for sensor 120, and Hall effect device 150 is attached between the pole pieces using an adhesive and/or also by encapsulating in plastic. Referring now to FIG. 14, the magnetic circuit for device 120 is shown in detail with lines of magnetic flux shown flowing from magnet 159, through pole piece 160 to combine with flux generated by magnet 158, across air gap 'B', through pole piece 142, across air gap 'C', through pole piece 144 and back across air gap 'B' to magnet 159. Unlike sensor 20, which employs only a single magnet mountable in either a north-south or south-north orientation depending on the desired polarity, magnets 158 and 159 must be mounted in rotor assembly 124 with opposite orientations; i.e., magnet 158 mounted north-south and magnet 159 mounted south-north (as shown) or both reversed. As rotor assembly 124 rotates about axis 126, a variable magnetic flux density is generated across air gaps 'B' and 'C' in accordance with the magnetic coupling of flux from the magnets 158 and 159 with the pole pieces 142 and 144. Similar to sensor 20, the polarity and flux density of the magnetic lines of flux in air gap 'C' is a function of the proportional magnetic surface area alignment between faces 154 and 156 of the stationary pole pieces and faces 167 and 166 of magnets 158 and 159, respectively. By incorporating two Alnico B magnets 158 and 159 in sensor 120, rather than the single magnet 58 of sensor 20, the measured magnetic flux density and corresponding output voltage of the Hall effect device is effectively doubled. Alternately, magnets 158 and 159 of sensor 120 may be constructed from an inexpensive, lower flux output magnetic material while maintaining the same measured magnetic flux density and corresponding output voltage of the Hall effect device of device 20. Referring to FIGS. 15A and 15B, another embodiment of a magnetic sensor 220 according to the present invention is shown. Rotational magnetic sensor 220 includes stationary magnetic fringing shields 268 and 269 in the air gap between the rotor and stator assemblies. Rotational magnetic sensor 220 includes a stator assembly 222. Stator assembly 222 includes stationary pole pieces 242 and 244 that span 120 degrees rather than the 180 span of, for example, pole pieces 42 and 142. Pole pieces 242 and 244 each include upstanding flanges 246 and 248, respectively, that define an air gap 'C'. Hall effect device 250 is adhesively attached between flanges 246 and 248 to measure the magnetic flux density in the air gap and produce an output signal corresponding to the flux density. Similar to sensor 120, sensor 220 includes a rotor assembly 224 having two magnets, 258 and 259, attached to a planar pole piece 260. However, magnets 258 and 259 span approximately 120 and 240 degrees, respectively, rather than 180 degrees as in sensor 120. Face 264 of pole piece 260 supports magnets 258 and 259 with face 266 of magnet 258 situated flush with face 267 of magnet 259 and parallel to pole faces 254 and 256. Unlike sensors 20 and 120, magnets 258 and 259 have a true semicircular profile since magnetic fringe shields are provided in sensor 220, rather than geometrically shaped magnets, as a means for compensating for non-linear effects. Magnets 258 and 259 are received on pole piece 260 centered about central axis 226. Similar to magnets 58, 158 and 159, magnets 258 and 259 generate axial lines of magnetic flux that are perpendicular to stationary pole faces 254 and 256 and perpendicular to faces 266 and 267. Magnetic fringing shields 268 and 269 are statically positioned in the air gap between the rotor and stator assemblies to reduce the non-linear magnetic effects that occur near zero flux density and maximum flux density positions of rotor assembly 224. Preferably, magnetic fringing shields 268 and 269 are insulated from and attached to the faces of pole pieces 242 and 244. In particular, magnetic fringing shield 268 is sized to span across the measured air gap 'C', and magnetic fringing shield 269 is sized to span approximately 120 degrees between the stator pole pieces 242 and 244. A linear analog waveform output signal is produced in the 180 degree span opposite magnetic fringing shield 269. Similar to sensor 120, magnets 258 and 259 must be mounted to rotor assembly 224 in opposite orientations or polarities; i.e., magnet 258 mounted north-south and magnet 259 mounted south-north or vice versa. As rotor assembly 224 rotates about axis 226, a corresponding magnetic flux density is generated in air gaps 'B' and 'C'. Similar to sensors 20 and 120, the polarity and flux density of the magnetic lines of flux flowing in air gap 'C' is a function of the proportional magnetic surface area alignment between faces 254 and 256 of the stationary pole pieces and faces 266 and 267 of magnets 258 and 259, respectively. As shown in FIGS. 15A and 15B, magnets 258 and 259 are aligned with equal portions of their respective faces across faces 254 and 256 of pole pieces 242 and 244, respectively (i.e., magnet 258 is bisected into two 60 degree portions relative to air gap 'C' and couples equal amounts of magnetic flux into air gap 'B', and magnet 259 has two 30 degree portions adjacent to ends of fringing shield 269 and couples equal amounts of magnetic flux into air gap 'B'). In the position shown, magnetic flux lines appear only in air gap 'B' and zero magnetic flux is found in air gap 'C'. In this "balanced" zero magnetic flux condition, the magnetic forces are balanced out in the magnetic circuit to define a magnetically neutral position, wherein rotation of rotor assembly 224 in either direction from this neutral position generates a magnetic flux having an assigned polarity across air gap 'C'. Conversely, when face 266 of magnet 258 and an equal portion of face 267 of magnet 259 are fully aligned with either faces 254 or 256 of stationary pole pieces 242 or 244, respectively, maximum levels of flux density appear in air gap 'C'. Referring now to FIGS. 16 and 17, a graph of the output voltage signal waveform 280, developed by sensor 220, is shown. In the 120 degrees spanned by opposite polarity maximum flux points 282 and 284, the signal waveform 280 includes a linear portion 281 and non-linear or slightly S-shaped portions 283 and 285. In the vicinity of the zero flux density point 286, magnets 258 and 259 are at their neutral position and magnetic fringing shield 268 prevents magnetic shunting from occurring to alter the output voltage waveform. In the vicinity of maximum flux points 282 and 284, magnets 258 and 259 are fully aligned with the stator pole pieces and magnetic fringing shield 269 sufficiently reduces the effects of magnetic coupling so that linear portion 281 is defined. In FIG. 17, the output voltage waveform 280 is depicted as a function of the rotation of rotor assembly 224. Linear portion 281 is shown in greater detail having a usable span of approximately three-fourths of the 120 degree span between maximum flux density points; i.e., approximately 88 degrees. In the linear portion 281, the output voltage signal ranges from approximately 0.75 volts to 2.5 volts linearly corresponding to angular position of rotor assembly 224, with respect to the stator, between approximately 16 degrees and 104 degrees. Referring to FIGS. 18A and 18B, another embodiment of a magnetic sensor 320 is shown according to the present invention. Sensor 320 includes geometrically shaped or contoured magnets 358 and 359 as well as a stationary magnetic fringing shield 369 in the air gap between the rotor and stator assemblies to compensate for the non-linear magnetic coupling and shunting effects and produce a linear output voltage waveform. Stator assembly 322 includes stationary pole pieces 342 and 344 that span 180 degrees similar to pole pieces 42 and 44 and pole pieces 142 and 144. Pole pieces 342 and 344 include upstanding flanges 346 and 348, respectively, that define an air gap 'C'. A Hall effect device 350 is adhesively attached between flanges 346 and 348 to measure the magnetic flux density in the air gap and produce an output signal corresponding to the flux density. Similar to sensor 120, sensor 320 includes a rotor assembly 324 having the two magnets, 358 and 359, attached to a planar pole piece 360 and centered about central axis 326. Face 364 of pole piece 360 supports magnets 358 and 359 with face 366 of magnet 358 situated flush with face 367 of magnet 359 and parallel to pole faces 354 and 356, respectively, of stationary pole pieces 342 and 344. As a means for compensating for non-linear effects, magnetic fringe shield 369 is provided geometrically shaped or contoured corresponding to the contour of geometrically shaped magnets 358 and 359. Referring now also to FIG. 19, magnetic fringe shield 369 includes openings 372 and 374 that permit axial lines of magnetic flux generated by magnets 358 and 359 to flow therethrough perpendicular to stationary pole faces 354 and 356, as indicated in the direction of arrows 372. The circumferential edge portions 376 and 378 of shield 369 substantially encompass the periphery of magnets 358 and 359 to prevent fringing of magnetic flux generated near the edges of the magnets. Magnetic fringing shield 369 is statically positioned in the air gap between the rotor and stator assemblies and includes a central portion 380 sized to span across the measured air gap 'C' to reduce the non-linear magnetic effects that occur between the rotor and stator assemblies as the sensor rotates through zero flux density and maximum flux density positions. Preferably, magnetic fringing shield 369 is insulated from and attached to the faces of pole pieces 342 and 344. In one specific embodiment, sensor 320 includes geometrically shaped magnets contoured similar to magnet 58 of sensor 20. In this specific embodiment, magnetic fringe shield 369 is matched to the geometric profile of magnet 358 and 359, wherein the inner radius 'R' of shield 369 is altered as a function of angular position 'P' according to Table 2: ANGLE (degrees) RADIUS (inches) (cm) P=90-100° R= 0.375 0.953 P=100-110° R= 0.373 0.947 P=110-120° R= 0.371 0.942 P=120-130° R= 0.367 0.932 P=130-140° R= 0.363 0.922 P=140-150° R= 0.359 0.912 P=150-160° R= 0.355 0.902 P=160-170° R= 0.351 0.892 P=170-180° R= 0.347 0.881 P=180-190° R= 0.343 0.871 P=190-200° R= 0.347 0.881 P=200-210° R= 0.351 0.892 P=210-220° R= 0.355 0.902 P=220-230° R= 0.359 0.912 P=230-240° R= 0.363 0.922 P=240-250° R= 0.367 0.932 P=250-260° R= 0.371 0.942 P=260-270° R= 0.373 0.947 P=270-280° R= 0.375 0.953

Claims: A magnetic rotational position sensor comprising: Hall effect means (50, 150, 250,350) for sensing magnetic flux and providing an output signal corresponding to a sensed level of magnetic flux; a rotor member (60, 160, 260, 360) arranged to rotate about a rotational axis (26, 126, 226, 326) and to conduct magnetic flux from a rotary surface of said rotor which is perpendicular to said axis of rotation, a stator member (22, 150, 250, 350) arranged to conduct magnetic flux from a static surface perpendicular to said axis of rotation, magnetic means (58, 158, 258, 259) for producing magnetic flux across a first air gap (B) between said rotary and said static surfaces, and the Hall effect means being arranged to sense the level of flux between said surfaces, wherein said rotor and stator members, said air gap (B), the magnetic means and the Hall effect means are located in series in a closely coupled magnetic circuit whereby the Hall effect means is able to provide an output which is a function of the magnetic surface alignment between said surfaces of the rotor and stator member, and the Hall effect means is located in a second air gap (C) of the stator member in series in a different part of said closely coupled magnetic circuit from said first air gap (B) and in a disposition to receive flux at right angles to said rotational axis characterised in that said stator member comprises first and second static pole pieces (42, 44) which include first and second upstanding flanges (46, 48) respectively, said first and second upstanding flanges defining said second predetermined air gap (C) therebetween. A magnetic rotational position sensor according to Claim 1, characterised in that said rotor member (24) includes upper and lower rotary pole faces (69, 64), said means for producing magnetic flux (58)is attached at a first face thereof to said lower rotating pole face (64) with an opposite second face (66) of said means for producing magnetic flux flush with said upper rotating pole face (69), and said second face (66) of said means for producing magnetic flux and said upper rotating pole face (69) are situated parallel to and spaced apart from said first and second stationary pole faces (54, 56) to define said first air gap (B). A magnetic rotational position sensor comprising: Hall effect means (50, 150, 250, 350) for sensing magnetic flux and providing an output signal corresponding to a sensed level of magnetic flux; a rotor member (60, 160,260, 360) arranged to rotate about a rotational axis (26, 126, 226, 326) and to conduct magnetic flux from a rotary surface of said rotor which is perpendicular to said axis of rotation, a stator member (22, 150, 250, 350) arranged to conduct magnetic flux from a static surface perpendicular to said axis of rotation, magnetic means (58, 158, 258, 259) for producing magnetic flux across a first air gap (B) between said rotary and said static surfaces, and the Hall effect means being arranged to sense the level of flux between said surfaces, wherein said rotor and stator members, said air gap (B), the magnetic means and the Hall effect means are located in series in a closely coupled magnetic circuit whereby the Hall effect means is able to provide an output which is a function of the magnetic surface alignment between said surfaces of the rotor and stator member, and the Hall effect means is located in a second air gap (C) of the stator member in series in a different part of said closely coupled magnetic circuit from said first air gap (B) and in a disposition to receive flux at right angles to said rotational axis characterised in that said rotor member (24) includes upper and lower rotary pole faces (69, 64),said means for producing magnetic flux (58)is attached at a first face thereof to said lower rotating pole face (64) with an opposite second face (66) of said means for producing magnetic flux flush with said upper rotating pole face (69), and said second face (66) of said means for producing magnetic flux and said upper rotating pole face (69) are situated parallel to and spaced apart from first and second stationary pole faces (54, 56) to define said first air gap (B). A magnetic rotational position sensor according to any one of Claims 1 to 3, characterised by a temperature compensating amplifier circuit including: a first amplifier stage (110) having a first input receiving said output signal corresponding to a sensed level of magnetic flux and defining a temperature dependent feedback path (R7, PTC) between a second input and an output thereof; and a second amplifier stage (108) having a first input connected to said output of said first amplifier stage, a second input connected to a temperature dependent offset circuit (104) output (105) and an output coupled to said temperature dependent feedback path and defining an output of said temperature compensating amplifier circuit. A magnetic rotational position sensor according to Claim 4, characterised in that said temperature dependent feedback path includes a resistor (PTC) having a positive temperature coefficient. A magnetic rotational position sensor according to Claim 5, characterised in that said temperature dependent offset circuit (104) includes a forward based diode (D1) producing a temperature dependent signal at said temperature dependent offset circuit output that varies in accordance with the voltage drop across said diode. A magnetic rotational position sensor according to Claim 6, characterised in that said diode is interposed in one leg of a wheatstone bridge circuit (R1, R2, R3, R4, R5, R6). A magnetic rotational position sensor according to Claim 4, wherein said output of said second amplifier stage (108) is resistively coupled to said second input (6) of said first amplifier stage (110). A magnetic rotational position sensor according to any one of Claims 4 to 8, characterised in that each of said first and second amplifier stages (110, 108) includes an operational amplifier. A magnetic rotational position sensor according any preceding claim, characterised in that said stator member (22) includes a static pole piece (42, 44) having a static pole face (54, 56) perpendicular to said rotational axis, and said Hall effect means (50)for sensing magnetic flux is attached to said static pole piece (42, 44). A magnetic rotational position sensor according to Claim 10, characterised in that said rotor member (24) includes a rotary pole piece (60) having a rotary pole face (69) perpendicular to said rotational axis, and said means for producing magnetic flux (58) is attached to said rotary pole face; and said rotary pole piece (60) is arranged to rotate relative to said static pole piece (42, 44) to align a portion of said rotary pole face (69) with a portion of said static pole face (54). A magnetic rotational position sensor according to Claim 11, characterised in that said means for producing magnetic flux (58) includes a geometrically shaped magnet contoured to reduce non-linear interactive magnetic coupling and shunting effects between said static pole piece and said rotary pole piece, said geometrically shaped magnet producing lines of magnetic flux normal to the static and rotary pole faces.

Inventors: LUETZOW ROBERT HERMAN, LUETZOW, ROBERT HERMAN
Applicants: KEARNEY NATIONAL INC, KEARNEY-NATIONAL, INC.
Application year: 1994
Main classification: G01D5
IPC codes: G01D5, G01B7, G01P3